A prior synchronous induction dynamo electric machine is disclosed in Sparrow U.S. Pat. No. 3,210,644, which is assigned to the assignee of the present application. Such prior dynamo electric machine includes a combination exciter and generator, each having relatively movable rotor and stator portions, with the rotor portions being mounted on a common shaft or separate shafts for rotation in response to a mechanical input by one prime mover or separate prime movers. The exciter stator is energized with an AC electrical signal, which induces a corresponding AC electrical signal in the rotating exciter rotor that is coupled to the generator rotor through a rectifier circuit to deliver a substantially DC signal thereto. The DC energized rotating generator rotor, which is of the salient pole type, induces an AC electrical output in the generator stator, which is coupled, for example, to an electrical load. Since the signal which effectively energizes the generator, i.e. that delivered from the exciter rotor to the generator rotor, is a DC signal, the rotational frequency of the exciter rotor is not particularly influential on the frequency of the AC electrical output from the generator stator; however, the frequency of such AC electrical output will be a function of the rotational frequency of the generator rotor and the number of poles of the generator.
In such prior dynamo electric machine at least a portion of the exciter stator is connected in shunt across a portion of the generator stator from which voltage electrical signal for energizing a portion of the exciter stator winding is derived. The remaining portion of the exciter stator winding also may be connected in series with the generator stator winding to receive the load current or a current proportional thereto and thereby to provide a compounding effect. Such a machine is described in greater detail in U.S. Pat. No. 3,210,644. The frequency characteristic of such AC electrical output, however, will depend on the rotational frequency of such mechanical input. Permanent magnets mounted in the generator rotor provide an initial magnetic field to commence production of at least a small AC electrical output upon start-up and which in turn aids the build up of rated or full voltage. Advantages of such machine include quick and automatic voltage regulation and recovery of voltage on sudden application of a load without requiring a separate regulator.
In some instances it is desirable that the AC electrical output from an electric generating machine have a substantially constant frequency, i.e. one that is substantially independent of the rotational frequency of the mechanical input thereto. In the past some machines for effecting such constant frequency AC electrical output have included electronic frequency monitoring feedback controls, such as frequency changers, that monitor the frequency of the AC electrical output from the generator stator and suitably adjust the frequency of an electrical signal that is delivered to energize the exciter stator. Such electronic controls increase the cost of such machines and in general tend to decrease the reliability thereof, as is well known.
U.S. Pat. No. 2,831,156 dicloses that a constant frequency AC electrical output can be produced by a double induction machine by supplying to the exciter of the machine a controlled frequency AC excitation signal from an external source and obtaining a net cancellation of the influence of mechanical rotational frequency on the frequency of the AC electrical output from the generator of the machine. The frequency of the AC electrical output, then, will correspond to that of such externally derived AC excitation signal. A drawback to such a system is the continuous need for the external frequency source to provide an input to the machine. The machine of such patent also uses an electronic voltage feedback circuit including a variable gain amplifier for controlling the voltage of the AC electrical output from the generator stator.
In induction motors slip frequency voltages are induced in the rotor circuits causing slip frequency currents to flow in the rotor circuits. Slip is defined, as follows: ##EQU1## where N.sub.s is synchronous speed of the rotating magnetic field on the stator equal to 120f/P; N is speed of rotor rotation; f is frequency in Hz; and P is pole number of the winding.
Moreover, in a wound rotor induction motor the rotor is normally wound with the same pole number as the stator; a squirrel cage rotor adapts itself to the stator magnetic field of any pole number so that the squirrel cage rotor pole number is the same as the stator pole number. The slip frequency currents flowing in the rotor conductors produce a magnetic field which rotates at a speed with respect to the rotor known as the slip speed sN.sub.s, which according to the above equation equals N.sub.s -N. Such magnetic field rotates in the same direction as the rotor. Thus, the magnetic field due to rotor currents rotates at the synchronous speed N.sub.s with respect to the stator or any stationary object of the machine; such field may be said to have a speed N.sub.s in space and would oppose the field created by the stator. The rotor field induces fundamental frequency voltages in the stator windings.
As the rotor field rotates at synchronous speed in space it continues to induce fundamental frequency currents in the stator, whether the rotor is of the wound rotor or squirrel cage type. A rotor load leads to a reduction in speed, i.e. an increase in slip, and generally results in larger rotor currents which are reflected by equivalent fundamental frequency currents in the stator windings being supplied from the input source to which the stator is connected.
The just-described principle of induction motor operation has been extended to double induction dynamo electric machines. In such a double induction machine, assuming the first (exciter) stator is supplied with currents (an AC excitation signal) of a given frequency, slip frequency voltages are generated in the first (exciter) rotor, as the latter rotates with respect to the former. The second (generator) rotor acts like a load on the exciter rotor. If both the exciter and generator are wound for the same number of poles, the resulting currents flowing in the second rotor produce a magnetic field in the generator which rotates at the same speed in space as the synchronous speed of the magnetic field of the first stator. This second magnetic field, then, generates voltages in the second stator winding at a frequency which is the ame as that of the AC excitation signal applied to the first stator winding.
The foregoing brief description of the principle of operation of a double induction machine presumes that the exciter and generator either have respective wound rotors that are coupled electrically in reverse phase sequence relative to each other and, for facility, are mounted on a common shaft for rotation at a same mechanical rotational frequency, or that a single squirrel cage rotor is associated with both the exciter stator and generator stator. These are the cases in the preferred embodiments of the present invention and also are the illustrated mechanical embodiments disclosed, for example, in the above-mentioned U.S. Pat. No. 2,831,156.